The invention is directed generally to a thermostat for measuring the temperature within a structure and controlling energization of the temperature modifying apparatus for the structure, and more particularly to a thermostat for use with any variable-capacity heating or cooling equipment including two-speed heat pumps, two-stage furnaces and other such equipment.
It is desirable to design thermostats for heating and/or cooling systems that control room temperature swings within the structure so that the occupants do not experience discomfort due to widely varying temperatures. The room temperature swing has been found to be a function of the heating/cooling capacity of the plant, the thermostat cycling rate, the response time or time constant of the plant, and the thermal load on the building. A room temperature swing of 2.degree. F. is generally considered the upper limit for acceptable comfort. By combining these factors, rules of thumb have been established for appropriate cycle rates for various heating/cooling systems: e.g., 6 cycles per hour for forced warm air furnaces, 3 cycles per hour for heat pumps and air conditioners. The variable-capacity plants mentioned in the first paragraph are able to modulate their heating/cooling output from full-capacity down to perhaps 50% of that value. At lighter loads, therefore, they can cycle on and off at low capacity resulting in a smaller room temperature swing. The cycle rate of the thermostat controlling this type of plant could be reduced from the typical values listed about without sacrificing comfort.
Balanced against the need for keeping the room temperature swing within the structure within desirable limits is the operational efficiency of the heating plant. It is desirable to maintain the lowest cycle rate possible yet insure that room temperature swings do not become so excessive as to compromise comfort. U.S. Pat. No. 4,356,962 to Levine uses a search method in order to adaptively achieve a predetermined temperature increase during a heating cycle and a thermal overshoot period which occurs following deenergization of a furnace. Levine's apparatus senses the change in building temperature, adaptively modifies either or both of the on and off cycles of the furnace and the delays associated with the furnace blower, and increments or decrements the on and off times of the furnace by one unit each burn cycle. Levine suggests that a unit may in the range of about 30 seconds or one minute or the like. However, since an on- plus off-cycle may typically be as long as an hour, it is believed that Levine's system may significantly lag actual temperature changes within the structure. Such a lag could result in an inability to reduce undesirable room temperature swings under rapidly changing conditions, such as might be experienced during the morning hours in many areas.
The invention operates to control room temperature swings within a desirable band through adjusting the anticipator time constant of a thermostat having an anticipator control. By adaptively varying this thermostat parameter, closed-loop temperature control is maintained at all times. In other words, the time periods of the on- and off-cycles are not calculated prior to the cycle--on the contrary, the thermostat remains responsive to load disturbances (or setpoint changes) throughout the cycle and can, therefore, initiate or terminate heating/cooling plant operation exactly when needed. The principal of the invention is to maintain the lowest cycle rate possible yet continuously control temperature in response to a temperature set point to insure that room temperature swings do not become so excessive as to compromise comfort.